How to Adjust the Hydraulic Injection Molding Machine Pressure?
Hydraulic injection molding machine pressure adjustment
When we are using the hydraulic injection molding machine, all movements in the injection molding process generate pressure. Only the appropriate control of the required pressure can produce a finished product of good quality.
The following actions of the machine are the main movement during injection molding process:
1 - Plasticizing process
The plasticization is most important steps which related to the product quality directly, in which the screw plays the most important role.
First, the screw must ensure that the material is melted and homogenized.
This process can be adjusted with back pressure to avoid overheating. The polymer can’t be rotated at an over-high speed in the melting zone of the screw. Otherwise, the polymer will degrade.
On the other words, each polymer has a different maximum rotational speed. If this limitation is exceeded, the molecules will stretch and the polymer backbone will break. However, the most important is the forward movement of the screw during the injection and after the holding.
The subsequent cooling process, including intrinsic stress, tolerances and warpage, is important to ensure product quality. This is all determined by the design and quality of the mold. It is related to the mold cooling runner and mold temperature control. But This does not interfere with mechanical adjustments.
Therefore, it can be concluded that the product quality is mainly determined by the action of the screw movement without considering the energy consumption and machine reliability, and the other additional conditions ( Such as the quality of the mold).
On hydraulic injection molding machines, this screw’s movement is realized by sensing the oil pressure. Specifically, the oil pressure activates a set of valves, in which the hydraulic oil is adjusted or released.
2 - Injection speed control
Injection speed control includes options like open-loop control, semi-closed-loop control and closed-loop control.
- The open loop systems rely on a common proportional valve. Proportional tension is applied to the desired proportion of the hydraulicoil, which causes it to create pressure in the injection cylinder. Then it further causing the injection screw to move forward at a speed.
- The semi-closed loop system uses a closed loop proportional valve. The loop is closed at the location of the closing port, which controls the flow rate of the oil by moving within the valve.
- The closed loop system closes when the screw translates speed. Besides, aspeed sensor (usually a potentiometer type) is used in a closed loop system to periodically detect tension. And the oil flowing out of the proportional valve can be adjusted to compensate for the speed deviation. Closed-loop control relies on dedicated electronics integrated into the machine. In addition, its pressure control can ensure uniform pressure during the injection and holding, as well as uniform pressure in every production cycle.
The proportional valve is adjusted according to the detected pressure value. Then, the deviation is compensated according to the setting pressure. In general, hydraulic pressure can be monitored.
Knowing the actual pressure the material can withstand also helps predict the actual weight and size of the plastic product based on the setting pressure and temperature conditions. In fact, by increasing the holding pressure, more material can be injected into the mold cavity. And it can reduce part shrinkage.
When near melting conditions, semi-crystalline polymers show large changes in specific volume.
Hydraulic components: output and pressure regulation
The average hydraulic pressure generated by the pump is 140 – 175 bar, which is particularly suitable for injection molding. At the other stages of the injection molding cycle, the requirement of the high pressure is significantly lower, except in certain cases which require rapid plasticization.
To reduce energy consumption, variable displacement pumps and accumulator can be used in the peak discharge periods. The fixed pumps which drive an equal amount of oil per time is out of day sooner or later.
The pump selection is determined by the amount of oil that needs to be drive at a given time. For the speed of the three-phase motor, it’s generally 1440 rpm. What’s more, when necessary it is required to install double pumps. But only in the plasticizing process (power up to 100%), the utilization rate of the oil pump reaches the maximum. During the standstill process, the machine does not need energy consumption. Even if it does, it is a power loss.
The use of valves
All injection molding machines use proportional servo valves. Two or more sets of proportional valves are installed on the injection molding machine in order to accurately control the following aspects:
- Clamping unit for mold opening speed (two stages or more), Clamping unit for mold closing speed (two stages or more), mold closing safety, injection (3 – 10 stages), suction and ejector ( two stages ).
- Mold opening pressure, mold closing pressure, mold safety, injection (one time in filling stage, 3 – 10 in subsequent stages), screw rotation speed (grade 3-5).
The electromagnetic valve, the weak input electrical signal is converted into a hydraulic output signal. And it is modified according to the required discharge requirements. The valve must have a fast, repeatable and low-hysteresis discharge response to tension or general commands.
In fact, the aim of the current research is to improve the frequency response to enable dialogue between power equipment (hydraulic side) operating at frequencies in the kilohertz (kHz) and electronic equipment.
Since effective discharge depends on the effect of the degree of polymerization (DP) on the valve. The oil temperature in the hydraulic circuit must be maintained in the range of 45-55°C (usually with a closed-loop regulation system). And the specific temperature depends on the fluid viscosity and the geometry of the transition port.
Without a proper adjustment system in the valve, the temperature rise will cause the viscosity of the hydraulic oil to decrease. If it is equipped with a balanced opening threshold, the output can be increased. Generally, increasing the output of hydraulic oil means faster for the injection speeds.
Precise control of high-tech servo-actuated valves essentially eliminates hysteresis and enhances repeatability of all functions.